Production of fine grained ingots for the advanced superalloys

ABSTRACT

The highly alloyed superalloys are cast as a well stirred twophase liquid/solid mixture into a heated mold and solidified with the maintenance of a well stirred two-phase layer preceding the axially advancing solidification front to provide a homogeneous ultra-fine-grain ingot.

United States Patent Moore et al.

[54] PRODUCTION OF FINE GRAINED INGOTS FOR THE ADVANCED SUPERALLOYS [72] Inventors: Joseph B. Moore, Jupiter Tequesta; Roy L.

Athey, North Palm Beach, both of Fla.

United Aircraft Corporation, East Hartford, Conn.

22 Filed: Jan. 20, 1971 21 Appl.No.: 107,908

[73] Assignee:

[52] US. CL... ..l64/l22, 164/60, 164/133 [51 Int. Cl. ..B22d 27/06, 822d 25/06 [58] Field of Search ..l64/47, 60, 77, 80, I22, I33

[5 6] References Cited UNITED STATES PATENTS 3,508,914 4/1970 Buehler ..l64/80 UX QQQGQQO @QQQOQQQ [151 3,669,180 [4 June 13, 1972 Athey and Moore, Development of lnlOO Turbine Blades, Feb. 28, 1968 pp. 1- 20.

Primary Examiner-J. Spencer Overhollser Assistant Examiner-John E. Roethel Attorney-Richard N. James [57] ABSTRACT The highly alloyed superalloys are cast as a well stirred twophase liquid/solid mixture into a heated mold and solidified with the maintenance of a well stirred two-phase layer preceding the axially advancing solidification front to provide a homogeneous ultra-fine-grain ingot.

1 Claim, 1 Drawing Figure PRODUCTION OF FINE GRAINED INGOTS FOR THE ADVANCED SUPERALLOYS BACKGROUND OF THE INVENTION The present invention relates to the alloy field and, more particularly, to the production of fine-grained cast ingots formed from the highly alloyed superalloys.

In a speech delivered at the annual meeting of the American Institute of Mining, Metallurgical and Petroleum Engineers (AIME) on Feb. 28, 1968, entitled The Development of IN100 Turbine Blades for the J58 Jet Engine, the present inventors described some of the difiiculties arising from the use of the advanced nickel base superalloys during the early development of the J58 engine. Engine operating experience had demonstrated that the state-of-the-art alloys available at the time were not satisfactory in the more demanding environment of the advanced gas turbine engines. Stronger alloys were obviously required.

With the stronger alloys, test engine operation revealed excessive airfoil growth in one case and cracked root serrations in another case. Creep tests in the range of l,300 l,500 F. revealed a lack of adequate third stage creep, this lack of duetility being responsible for premature and unpredictable stress rupture failure at the blade root. In addition to the strength and ductility problems an excessively broad scatter of mechanical properties was evident due to uncontrolled grain structure. This scatter was incompatible with predictable engine operating requirements.

In the course of a development program designed to establish casting techniques for providing uniformity and reproducibility of mechanical properties in advanced superalloy cast structures, such techniques were in fact developed to produce favorable mechanical properties together with reproducibility of result. In particular, techniques were established for effecting a controlled equiaxed grain structure in the castings.

The method utilized involved manipulation of the melt from which blade castings were to be made to obtain random nucleation in the melting crucible and grain growth therefrom in the blade mold during solidification. In the process after sufficient holding time to obtain complete melting, the heat input to the crucible was lowered at a rate designed to minimize the thermal gradients in the melt while maintaining maximum stirring effect. Upon reaching the point where incipient nucleation was observed, the crucible heat was increased sharply to increase the temperature of the melt to the pour point. Even with this temperature increase, the nuclei formed during the cool-down portion of the cycle persist for a reasonable time. With the imposition of a proper power-time relationship to assure persistence of the nuclei at the casting temperature, and with randomness of the nuclei being assured by the stirring action associated with the increase of heat to the crucible, the melt was poured into the blade mold with grain growth proceeding from the dispersed nuclei with solidification producing an equiaxed microstructure.

The above technique was found to be preferable to other methods such as surface innoculation, surface innoculation resulting in a structure wherein the equiaxed grain condition is only superficial.

SUMMARY OF THE INVENTION The present invention comprises an improved method of providing castings characterized by an equiaxed grain structure and further characterized by a fine grain size. It is applicable to the superalloys exhibiting sufficient strengths to limit creep to a maximun of 0.1 percent in 150 hours at 1,300 F. under a load of 80,000 p.s.i.

Briefly stated, the present invention contemplates the pouring of a well stirred melt in a two-phase solid/liquid condition into a heated mold and solidifying the same in the presence of a reservoir of a well stirred reservoir of the melt. Accordingly, it comprises an improved slush casting technique which is particularly applicable to the production of the larger size ingots from the advanced, highly alloyed superalloys including the nickel-base superalloys.

BRIEF DESCRIPTION OF THE DRAWING The drawing depicts ingot mold apparatus for the production of the ultra-fine-grain cast ingots described herein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Alloys of the type to which the present process is particularly applicable are those advanced superalloys characterized by high creep strengths including such alloys as those shown in U.S. Pat. No. 3,061,426, having a representative chemistry as follows: (by weight) 10 percent Cr, 15 percent Co, 4.5 percent Ti, 5.5 percent A], 3 percent Mo, 0.17 percent C, 1 percent V, 0.015 percent B, 0.06 percent Zr. Other alloys of the same general type include: Alloy A 9 percent Cr, 10 percent Co, 2 percent Ti, 5 percent A], 7.8 percent Mo, 12.5 percent W, 0.15 percent C, 1 percent Cb, 0.015 percent B, 0.05 percent Zr; and Alloy B 15 percent Cr, 15 percent Co, 3.4 percent Ti, 4.3 percent Al, 4.4 percent Mo, 0.07 percent C, 0.02 percent B.

The fundamental objective of the present invention is the provision of sound, homogeneous ultra-fine castings from the superalloys. This requires in the casting process not only the establishment of many sites for grain growth (nuclei) uniformly distributed through the melt in the casting mold but also accommodation of such factors as thermal shrinkage which is unaccommodated can lead to the generation of voids or pipe.

Suitable apparatus for the generation of the desired castings is shown in the drawing. In the particular apparatus illustrated, a lower mold 2 with integral hot top 4 is positioned on a copper stool 6. The lower mold is 16 inches in diameter and 24 inches high, these dimensions corresponding to the desired ingot size with the hot top being 8 inches in diameter and 12 inches high. The copper stool on which the mold assembly rests is 24 inches in diameter and 18 inches high with a 6-inch base. Provision for temperature control of the copper stool is made, although not shown.

The lower mold 2 comprises a nonmagnetic stainless steel shell 8 containing a suitable ceramic mold 10 of the desired configuration, in the present case essentially cylindrical. Surrounding the lower mold, external of the stainless steel shell is a stirring coil 12 which is utilized for maintaining a homogeneous distribution of the nuclei from which grain growth occurs.

The upper mold 4 is somewhat similarly constructed comprising a stainless steel shell 14 with suitable ceramic insulation 16 internal thereof. In addition the stainless steel shell is externally surrounded by a graphite susceptor l8 and external of the susceptor by a stirring and heating coil 20.

In the casting process, the alloy to be cast is melted in a crucible, with sufficient time at temperature to assure complete melting. The temperature of the crucible is slowly lowered to provide a two-phase mixture of liquid and solid accompanied by sufficient stirring action to minimize thermal gradients in the melt and a homogeneous distribution of the fine solid particles or nuclei of the melt.

The two-phase mixture or slush is then poured from the crucible into the desired mold, for example of the type depicted in the drawing, the mold being preheated to a temperature below the solidus temperature of the alloy but preferably close thereto. In the case of the nickel base superalloys of principal interest, preheat temperatures of 1,900 F. are satisfactory.

As the alloy solidifies shrinkage takes place tending to result in the generation of a void internal of the casting. The slush thereover is continually stirred and kept in a condition of sufficient fluidity to accommodate this shrinkage as complete diametrical solidification proceeds upwardly in the mold. Thus the slush in the upper portion of the lower mold compensates for the shrinkage occurring beneath it.

Heating and stirring is provided in the mold hot top and, in effect, provides not only a makeup reservoir for the lower mold but permits control of the viscosity of the material both in and below it. Regulation of heat in the hot top is further provided to assure that solidification through the ingot proceeds from the well stirred slush as solidification occurs upwardly.

The net result is the provision of an ultra-fine-grain equiaxed structure characterized by the substantial absence of segregation and voids.

Although the invention has been described in detail with respect to certain preferred embodiments, it will be understood that these are illustrative only for numerous modifications will occur to those skilled in the art.

What is claimed is:

1. In the casting of superalloys characterized by strengths at l,300 F. sufficient to limit creep to a maximum of 0.1 percent in 150 hours under a load of 80,000 p.s.i., the improvement which comprises:

melting the superalloy in a crucible;

slowly reducing the temperature of the crucible to a level below the nucleation temperature of the superalloy and providing a substantially homogeneous liquid/solid slushy mixture;

providing a casting mold having upper and lower zones, the upper zone being temperature controllable independently of the lower mold;

preheating the lower portion of the mold to a temperature below the solidus temperature of the superalloy;

pouring the mixture into the preheated mold, an excess of the mixture provided in the upper zone thereof sufficient to cover the volume loss associated with shrinkage occurring as a result of solidification;

and maintaining a zone of the homogeneous two-phase mixture in advance of the solidification front until the ingot of the desired size is attained. 

